Extreme ultraviolet light (EUV light) will in the future be used more extensively in mass production lithography for manufacturing semiconductor structures and components with structure sizes of less than 70 nm. For this purpose, radiation with a wavelength of approximately 13 nm, in particular, will be used. For the operation of the respective production systems, narrow-band spectral filters with sufficient service life will be needed. These are necessary, since all sources of extreme ultraviolet light known to the art up to now have a very low degree of efficiency which is expressed, on the one hand, in a high heat load and in debris, and, on the other hand, in a large and undesirable share of radiation outside of the radiation band needed for lithography with extreme ultraviolet light. Debris here is understood as any material, e.g. ceramic materials or metals, which, due to the high input of energy for the generation of the extreme ultraviolet light, is finely pulverized and then deposited on the optical elements and, for example, in the lithographic chamber in the form of a layer. By using a filter as separator between the radiation source and the lithographic apparatus, the debris can be kept out of the lithographic apparatus, as described in the WO 00/41875. It is only necessary to exchange the filter when the debris has accumulated on the filter to a degree that transmission is no longer sufficient.
Optical filters for the infrared, the visible, and also the ultraviolet wavelength range have been known to the art for a long time. As a rule, this involves the equipment of transparent substrates with a layer having a wavelength-dependent transmission rate, i.e. the transmission, starting from a particular wavelength, increases or decreases at a steep angle. Layers with this type of transmission edges are often used as optical low-pass filter layers, since they only allow light to pass from a particular wavelength on or within a particular wavelength range.
Particularly for the ultraviolet wavelength range, various filters used, for example, in sunglasses or in sun beds are known to the art.
For example, from the application EP 0 267 655 A2, a filter is known to the art which is manufactured of foil of synthetic material with UV-absorbing pigments, wherein the foil of synthetic material is transparent at 320 to 400 nm and absorbs at 290 to 320 nm, so that only the relatively harmless UVA radiation is allowed to pass.
From U.S. Pat. No. 5,182,670, a narrow-band filter for ultraviolet light comprising at least two AlxGa(l−x)N layers with differing aluminum content and optional thickness is known to the art. By varying the aluminum content, the energy gap of this semiconductor is changed, and thereby the wavelength of the transmitting light in the range of 270 to 365 nm. By combining at least two layers, transmission and reflection can be tuned to fit the system.
With the low-pass filter for the UV range described in U.S. Pat. No. 5,978,134, the plasmone frequency of metal is used as limit frequency for the reflection. For this purpose, earthy base or alkali metals are used above all. Furthermore, an interference system of MgO or Al2O3 and MgF2, or of Al2O3 and SiO2 may be applied on the low-pass filter. The application DE 44 10 275 A1 discloses a thin-layer band pass filter for the ultraviolet wavelength range which is transparent at 320 to 430 nm. The filter edge is implemented by an absorption edge of the material of a thin layer, and the spectral position thereof is established by adjusting the composition of the material of the thin layer. In this process, one component of the material generates an absorption edge above the desired spectral position, the other component, an absorption edge below the desired spectral position. Preferred materials are oxides, fluorides, sulfides, and oxinitrides of metals, in particular, Ta2O5, Nb2O5, TaOxNy, HfOxNy, ZrOxNy, TiO2, and ZnS. Particularly preferred are Nb2O5/Ta2O5 single layers with a thickness of between 53.8 nm and 85 nm on SiO2 layers of 53 to 138 nm thickness. These layer systems have repeat factors of between 1 and 5, in order to make it possible to utilize, in addition, interference effects.
In the extreme ultraviolet wavelength range, reflecting multi-layer systems, particularly those composed of molybdenum and silicon layers, are used. These have the disadvantage that their production is very complicated and cost-intensive. Therefore, these types of mirrors have to be protected against undesirable spectral shares in order to decrease the radiation load and thereby to increase the service life. This requirement is reinforced by the fact that the mirrors reflect radiation with photon energies of <10 eV with undesirably high efficiency. Furthermore, the contamination by debris also very quickly makes the mirror unusable. This means that very high costs and high standstill times are incurred which are unacceptable for the use of the EUV lithography for mass production.